Nowadays, tomographic X-ray imaging technology has found wide acceptance in different fields such as clinical diagnosis, industrial inspection and security screening. In the clinical field, dedicated high-resolution CT imaging systems have recently emerged as important new tools for cancer research, which is because CT scanners are an essential medical image modality to non-invasively examine a region of interest (such as e.g. the interior organs, the cardiovascular system and/or any other anatomical or pathological structures) in the interior of a patient's body. A tomographic imaging system thereby acquires a series of 2D projection images from a number of distinct projection directions around the patient which may then be used for creating a three-dimensional reconstruction of an anatomy to be visualized.
Recently, with use of spiral CT, the new generations of CT scanners, radiologists are able to save much time by rapid examination of a patient during a single breath-hold, thereby utilizing the 3D imaging capability of today's rotational CT imaging systems. In a conventional isocentric 3D rotational X-ray scanner system of the rotary gantry type as used in multi-slice spiral CT, a detector array sensitive to X-radiation is irradiated with a fan- or cone-shaped X-ray beam emitted by an X-ray tube diametrically arranged with respect to said detector array, wherein both said X-ray tube and said detector array are placed on a rotational gantry that is continuously rotated around the patient. To acquire a set of 2D projection images which can be used to reconstruct a three-dimensional image of an anatomy volume in the interior of a patient's body to be non-invasively examined, the X-ray tube and detector array are rotated along a circular trajectory around the patient's body while the patient is lying on a patient table which is advanced along the axis of rotation.
Area-beam detector 3D imaging systems have operated by rotating an X-ray tube and a detector in circular paths around a central axis of rotation. The axis of rotation is positioned to be at the center of a region or volume of interest of a patient anatomy. An X-ray tube and an X-ray detector, such as an image intensifier, are typically mounted at opposite ends of a rotating C-arm support assembly. The X-ray tube irradiates a patient with X-rays that impinge upon a region of interest (ROI) and are attenuated by internal anatomy. The X-rays travel through the patient and are attenuated by the internal anatomy of the patient before the attenuated X-rays then impact the X-ray detector. 3D image data is acquired by taking a series of images while the X-ray tube/C-arm/detector assembly is rotated about the axis of rotation on which the region of interest within the patient is centered. A plurality of two-dimensional (2D) cross-section images are processed and combined to create a 3D image of an object being scanned.
Conventional mobile C-arm assemblies utilize simple support structures and geometries to mount the X-ray tube and X-ray detector on the C-arm. The support structure holds the X-ray tube and detector on the C-arm and maintains a predetermined, constant distance between the X-ray tube and X-ray detector. Thus, the distance between the X-ray tube and the axis of rotation and the distance between the detector and the axis of rotation remain constant and fixed.
In current C-arm X-ray fluoroscopy imaging systems, a 3D tomographic image reconstruction may be performed by sweeping the C-arm in a semi-circular arc around an object of interest. Using cross-arm motion, the arc is circular and therefore isocentric. For example, using a C-arm, an X-ray beam may be swept around a head of a patient (e.g., a CT scan in a circular arc around the head). The volume image reconstruction is performed through 2D projection scan images. Sweeps are accomplished on cross-arm motion with the C-arm positioned at the head of a table sweeping around the head of the table. Thus, the object stays at the center (isocentric motion).
Irrespective of the applied technology, three-dimensionally reconstructed images of an object of interest which are calculated from a set of 2D projection images acquired from a number of distinct projection directions by a rotational CT scanner are often severely distorted by CT artifacts, given by the difference between the detected intensity values in an acquired CT image and the expected attenuation coefficients of the object to be visualized. This is due to multiple reasons which will briefly be explained in the following section. These artifacts, which are still persist in spiral CT as in conventional tomographic X-ray imaging, may enormously degrade the image quality of a reconstructed CT image and play an important role in diagnostic accuracy. Unfortunately, it is not always possible to say if there actually exists an artifact in a CT image because this often depends on a radiologist's judgment. In case of severe artifacts, however, physicians are often not able to give a reliable diagnosis, which is because the anatomies of interest may be hidden or completely distorted.
In general, CT artifacts can be classified into four categories: a) physics-based artifacts including beam hardening, photon starvation and undersampling artifacts, b) patient-based artifacts including metallic and motion artifacts, c) scanner-based artifacts including artifacts which are caused by detector sensitivity and mechanical instability (which is the type of artifacts to which the present invention is dedicated) as well as d) spiral-based artifacts which arise due to spiral interpolation. Careful patient positioning, avoiding patient motion and optimum selection of scanning parameters are thus important factors in avoiding CT artifacts.
The vast majority of artifacts in CT images appears as streak effects and may be caused by metallic objects, beam hardening, photon starvation and/or object motion. If a measurement value of one detector channel in a single reading is disturbed, a single streak occurs. If one channel drops out over a full rotation, which is the case if an X-ray detector of a third-generation rotational CT scanner system is out of calibration, the detector will give a consistently erroneous reading at each angular position, hence resulting in a circular ring artifact.
Another important reason for the emergence of ring artifacts consists in the fact that conventional C-arm based 3D rotational X-ray scanner systems for use in computed tomography that are capable of performing rotational scans for three-dimensionally reconstructing an object of interest, such as e.g. an anatomical region in the interior of a patient's body to be non-invasively examined by means of tomographic X-ray imaging, may not be perfectly isocentric. In practice, mechanical bending and play as well as imperfect aligning of mechanical components may cause the “center of rotation” to vary over the rotation angle. Many conventional C-arm systems in use are unable to perform an exact 3D tomographic reconstruction with an orbital motion of the C-arm because the trajectories of the X-ray tube and detector array may not be perfectly isocentric. As a consequence, acquired 2D projection images of an object of interest to be three-dimensionally reconstructed are distorted due to the non-isocentric imaging arc and are unusable for clinical, diagnostic or interventional purposes. Although circular ring artifacts would rarely be confused with a pathological structure, they can severely impair the diagnostic quality of a tomographic image. For accurate ring artifact detection and correction, it is thus of essential importance that the ring artifact center, which constitutes the effective center of rotation, is known very accurately, which means to a submillimeter level of accuracy down to the 3D volume voxel size, especially for soft tissue imaging. This center position may then be used for eliminating ring artifacts in rendered 3D reconstructions of an object to be visualized. A calibration system and method which improve tomographic image reconstruction by eliminating circular ring artifacts which may arise due to non-isocentric motion of the X-ray tube and detector array when acquiring 2D projection images of an object by means of a not perfectly isocentric 3D rotational X-ray scanner system would hence be highly desirable.
US 2005/0084147 A1 proposes a method for three-dimensionally reconstructing an object of interest based on a set of 2D projection images which are acquired along a non-isocentric trajectory. As described in this document, said method comprises the steps of determining and varying a distance between an object and at least one of an X-ray tube and an X-ray sensitive detector to form a virtual isocenter, maintaining an object at said virtual isocenter during the imaging of said object, normalizing a magnification change in image data obtained while said virtual isocenter is maintained and reconstructing an image of said object based on said image data and said normalized magnification change. The herein disclosed method may further comprise the step of moving a support including the X-ray detector and an X-ray tube in a non-circular arc to move the detector and the tube around the object while varying the distance between the detector and the object.